Raman Spectroscopy

Shine a laser at almost anything, and nearly all the light that bounces off comes back at exactly the same colour it arrived with. But about one photon in ten million comes back very slightly changed: a little lower in energy, or occasionally a little higher. Those rare, altered photons carry a detailed fingerprint of the molecule they touched. Measuring them is Raman spectroscopy, a technique that can identify a white powder through a plastic bag, check a painting’s pigments without touching it, and analyse rocks on Mars.

The discovery

In 1928, the Indian physicist C. V. Raman, working in Calcutta (now Kolkata) with K. S. Krishnan, observed that light scattered by liquids contained faint additional colours not present in the original light. Raman received the 1930 Nobel Prize in Physics for the discovery, the first Nobel science prize awarded to an Asian scientist. The date the effect was announced, 28 February, is celebrated as National Science Day in India.

For decades, Raman spectroscopy was limited by weak signals. The invention of the laser in the 1960s, with its intense, single-wavelength light, turned it into a practical, powerful technique.

Three kinds of scattering

When light of a single frequency hits a molecule, the photons can be scattered in three ways:

1. Rayleigh scattering (elastic): the photon leaves with the same energy it arrived with. This is by far the most common, and it’s the same process that makes the sky blue.
2. Stokes Raman scattering (inelastic): the molecule takes a little energy from the photon and ends up in a higher vibrational state. The scattered photon has lower energy (longer wavelength).
3. Anti-Stokes Raman scattering: a molecule that was already vibrating gives some energy to the photon. The scattered photon has higher energy (shorter wavelength). Anti-Stokes lines are weaker because fewer molecules start in excited vibrational states at room temperature.

The energy difference between the incident light and the Raman-scattered light equals the energy of a molecular vibration. That’s why Raman spectroscopy, like infrared spectroscopy, is a form of vibrational spectroscopy.

The Raman spectrum

A Raman spectrum plots intensity against Raman shift, the difference in wavenumber between the scattered light and the laser light, in cm⁻¹. The Raman shift is independent of the laser wavelength used, so peaks from different instruments can be compared.

Peaks correspond to vibrations of particular bonds and groups. For example:

Vibration	Approximate Raman shift (cm⁻¹)
C–H stretch	2800–3100
C≡C, C≡N stretch	2100–2260
C=O stretch	1650–1750 (often weak in Raman)
C=C stretch	1600–1680 (strong in Raman)
aromatic ring breathing	about 1000
S–S stretch	450–550
diamond (C–C lattice)	1332
graphite / graphene (G band)	about 1580

Raman vs infrared: complementary techniques

Both techniques probe molecular vibrations, but they follow different rules:

• A vibration is IR active if it changes the molecule’s dipole moment.
• A vibration is Raman active if it changes the molecule’s polarisability (how easily its electron cloud can be distorted).

As a result:

	Infrared	Raman
Strong for	polar bonds: O–H, C=O, N–H	non-polar, symmetric bonds: C=C, C≡C, S–S, C–C
Water	absorbs strongly, a big problem	scatters weakly, so aqueous samples are easy
Glass and plastic containers	absorb IR	usually transparent, so you can measure through them
Sample preparation	often minimal (ATR)	usually none
Main interference	water and CO₂ in air	fluorescence from the sample

In molecules with a centre of symmetry, such as CO₂, vibrations that are IR active are Raman inactive and vice versa (the mutual exclusion rule). The symmetric stretch of CO₂ doesn’t change its dipole, so it’s invisible in IR but strong in Raman; the asymmetric stretch is the opposite. Using both techniques gives a much fuller picture of a molecule’s vibrations.

The instrument

A Raman spectrometer has:

1. A laser, commonly at 532 nm (green), 785 nm (near-infrared) or 1064 nm.
2. Optics to focus the laser onto the sample and collect scattered light.
3. A filter to block the overwhelming Rayleigh-scattered light at the laser wavelength.
4. A spectrograph (diffraction grating) to spread the scattered light by wavelength.
5. A sensitive detector, usually a cooled CCD camera.

Coupling the spectrometer to a microscope gives Raman microscopy, which can analyse spots just a micrometre across and build chemical maps of a surface.

Challenges

• Weak signal: Raman scattering is inherently very weak, so good lasers, filters and detectors are essential.
• Fluorescence: some samples fluoresce under the laser, producing a broad glow that can swamp the Raman peaks. Using a longer-wavelength laser (785 or 1064 nm) often reduces this.
• Sample heating: intense lasers can heat, burn or change delicate samples, so power must be controlled.

Enhanced techniques

• Surface-enhanced Raman spectroscopy (SERS): molecules adsorbed on roughened gold or silver nanostructures can give Raman signals enhanced by factors of a million or more, enough in some cases to detect single molecules. SERS is used for trace detection of drugs, explosives and biomolecules.
• Resonance Raman: choosing a laser wavelength that matches an electronic absorption of the molecule greatly strengthens certain vibrations, useful for studying coloured molecules like haem in haemoglobin.

Uses of Raman spectroscopy

• Pharmaceuticals: checking the identity of raw materials through sealed packaging; detecting counterfeit medicines; distinguishing different crystal forms (polymorphs) of a drug.
• Forensics and security: identifying unknown powders and liquids, including suspected drugs and explosives, often through bags or bottles with handheld devices.
• Art and archaeology: identifying pigments and minerals non-destructively.
• Carbon materials: Raman is the standard way to characterise graphene, carbon nanotubes and diamond, and to tell natural from synthetic diamonds.
• Geology and mineralogy: identifying minerals, including on Mars, where rover instruments have used Raman spectroscopy to analyse rocks.
• Biology and medicine: research into non-invasive diagnosis, such as distinguishing cancerous from healthy tissue.
• Semiconductors: measuring stress and composition in silicon devices.

A worked example: calculating a Raman shift

A 532.0 nm laser produces a Raman-scattered peak at 562.4 nm. What is the Raman shift?

• Laser wavenumber = 1 ÷ (532.0 × 10⁻⁷ cm) = 18,797 cm⁻¹
• Scattered wavenumber = 1 ÷ (562.4 × 10⁻⁷ cm) = 17,781 cm⁻¹
• Raman shift = 18,797 − 17,781 = 1016 cm⁻¹

A shift of about 1000 cm⁻¹ is typical of the “breathing” vibration of a benzene ring, suggesting an aromatic compound. An infrared spectrum of the same sample would help confirm it.

Key takeaways

• Raman spectroscopy measures the tiny fraction of laser light scattered with changed energy by molecular vibrations.
• The Raman shift (in cm⁻¹) is characteristic of particular bonds and groups.
• Raman is strong for symmetric, non-polar bonds; infrared is strong for polar bonds, so the two are complementary.
• Water and glass interfere little, so samples can be measured in solution and through containers.
• Uses range from checking medicines and identifying drugs to analysing art, carbon materials and Martian rocks.